Plant growth depends on cell proliferation, expansion and differentiation. The final organ size is determined by the resulting cell number and cell size. Larger organs tend to contain more cells than smaller organs. The organ size and cell numbers are primarily controlled by internal developmental signals in plants. Some signals regulate cell growth and cell division, whereas others contribute to cell expansion and differentiation. Plant cell size is also correlated with nuclear size and thus with genome size and gene dosages. Many high-ploidy crop plants, such as rapeseeds, cotton, wheat and banana produce larger flowers, grains/seeds or fruits than those of diploid counterparts.
The domestication of many plants has correlated with dramatic increases in yield. Most phenotypic variation occurring in natural populations is continuous and is effected by multiple gene influences. The identification of specific genes responsible for the dramatic differences in yield, in domesticated plants, has become an important focus of agricultural research.
Two kinds of genes have been found in plants that regulate plant growth and development. Some genes can enhance plant growth while others suppress plant growth. For example, during leaf development, growth enhancing genes are active to keep young leaf growing. When the leaf reaches the full-size, the growth suppressing genes are activated to stop leaf from further growth.
One family of genes associated with plant growth that relate to improved yield in crops, the (ARGOS) Auxin-Regulated Gene involved in Organ Size gene is inducible by auxin (U.S. patent application Ser. No. 11/692,977, filed Jun. 15, 2005). Transgenic plants expressing ZmARGOS show a positive impact on biomass accumulation and rate of maize plant growth, as well as an increase in organ size. These maize genes will find utility for enhancing agronomic traits in maize (and other crops).
Another gene family, the CNR (Cell Number Regulator) genes (U.S. patent application Ser. No. 11/153,071, filed Mar. 29, 2007) can suppress plant growth. Transgenic plants expressing CNR show a smaller phenotype than the non-transgenic control. These genes show expression in diverse tissues, control cell number throughout the plant, and may play a role in regulating overall plant growth and vigor not simply one or a set of tissues.
In the case of Zm-D8 MPL (U.S. patent application Ser. No. 11/736,615, filed Apr. 18, 2007), flowering related filaments appear to be preferentially shortened such that the anthers are shorter than the stigma. Zm-D8 MPL constructs had significant shorter average root lengths and significantly fewer root tips per plant than the control plants. Thus, the gene is involved in the control of root architecture, particularly root length and root branching.
Accordingly, strategies requiring promoters to control the gene expression may be tried using promoters with a broad developmental expression pattern. These may be among the so-called constitutive promoters, one example being the maize ubiquitin promoter. Promoters could also be used to focus expression in one or several tissues (if desired enhanced cell number is deemed to be sufficient within a limited spectrum of the plant development). Such enhanced tissues of interest include for example, roots (enhance root development by diminishing gene or gene expression there), embryos (larger embryos, including effecting higher oil content of the whole seed), seedling (seedling vigor, enhanced mesocotyl size and extension to emerge more successfully from the soil), silks (enhanced silk emergence, including during drought conditions or to nick or synchronize with pollen donation), stalks (to increase the girth leading to greater stalk strength) and other plant tissues of interest.
In the past, when transgenes were used to increase the ear size or seed size and numbers, the whole plant size was also increased, resulting in low planting density in the field. The goal to increase grain yield is not yet achieved using this transgenic approach. The present invention can overcome this problem by using a second component gene that can suppress the over growth of vegetative tissues and other non-harvestable tissues. In maize, for example, the result will be that ear size and seed size and numbers are increased while the whole plant size remains the same or even becomes smaller. The photo-synthates used to increase ear size and seed size and numbers come from suppressing the growth of non-harvested tissues and organs such as tassel/pollen and stalk. In maize, male sterility can increase grain yield (Feil, et al., (2003) Euphytica 130:163-165). Detasseling without hurting leaves also increases grain yield by 6%. Thus, using growth suppression genes to reduce the energy used by male flower/pollen can increase grain yield. The optimum reduction for male flower/pollen is 80%. Only 20% of normal corn pollen is required to produce seeds.
The component genes have transgenic efficacy when expressed individually, ARGOS genes enhance overall plant growth and CNR and D8mpl suppress overall plant growth. In the stacked transgenic plants, although both genes are transgenically expressed in multiple organs, the growth-enhancing gene (e.g., ARGOS and growth-suppressing gene (e.g., CNR) may have different efficacy in different organs. For example, ARGOS may enhance growth more effectively in the ear than in other organs, and CNR may have stronger growth suppression in the tassel or plant height than elsewhere. Such differential effects of the stacked transgenes will produce transgenic plants with architectures that are different from those of plants with single transgene and are more desirable for agricultural needs. For example, the growth of reproductive and harvestable organs (e.g., ear) may be enhanced by the growth-enhancing gene (e.g., ARGOS) and that no-harvestable organ (tassel, plant stature) may be reduced by growth-suppressing gene (e.g., CNR). The same concept can be applied to other stacked transgenics. Depending on the consequences of the combined transgene effects (gene-gene interaction, gene-tissue interaction), resulted plant architectures from the stacked transgenes may be more or less desirable.
The present invention provides methods to increase crop yield utilizing transgenic complementary paired genes controlling maize growth and yield. The specific genes increase female reproductive organs and are paired with genes responsible for modifying the growth of non-yield specific plant tissues. Plants, plant progeny, seeds and tissues created by these methods are also described.